Tires have a complex layered structure in which various types of materials, such as rubber, chemical fibers, and steel cords, are layered. The supporting surface (tread surface) of a tire having a complex layered structure needs to maintain the uniformity of the tire radius and to suppress undulation (run-out) on the supporting surface in order to prevent radial run-out due to variations in the tire radius. On the other hand, irregularities called bulges or indentations may occur on the sidewall surface in addition to run-out and also need to be suppressed.
Accordingly, in a tire manufacturing process, it is necessary to inspect the form of the tread surface and the sidewall surface of a tire and to evaluate the inspected form.
Therefore, in a final tire manufacturing process (in an inspection process after vulcanization of tires), measurement of run-out values of the tread surface and inspection for a defective form of the sidewall surface are specifically performed. The tread surface has grooves, and the sidewall surface has intended projections due to characters or patterns formed thereon. Recently, there has been a demand for a means to measure the form of a tire without being affected by grooves or projections.
In recent years, in a technique for measuring run-out values of a tire, attempts have been made to measure run-out values by using a laser distance sensor, a three-dimensional form measuring device, a camera, or the like and to automatically evaluate the tire from the measured run-out values.
PTL 1 discloses a form measuring device that measures the form of a tire without a need to select a measurement line even if unnecessary irregularities are present on the surface of the tire, by obtaining sample data for one line on the tire using an optical displacement meter and removing a predetermined signal pattern from the obtained sample data.
PTL 2 discloses a technique in which numeric data for one rotation of a tire is acquired by scanning the tread surface of the tire in the circumferential direction using a non-contact displacement meter, and, if the difference between numeric data at a position of interest and the median of a group of a plurality of pieces of numeric data at positions before and after the position of interest is larger than a threshold, the numeric data at the position of interest is determined to be noise.
PTL 3 discloses the following technique. The amount of displacement due to irregularities on the tire surface on two lines is measured on the sidewall surface, one of the two lines passing through a position in an intended projection, the other not passing through any position in any intended projection, and original waveforms A and B of the two lines are generated. Next, approximate curves A1 and B1 that represent the undulation components of the generated original waveforms A and B are generated, the approximate curves A1 and B1 are subtracted from the original waveforms A and B, and irregular waveforms A2 and B2 are generated respectively. Next, the irregular waveform A2 is multiplied by the irregular waveform B2, and an irregular form from which an intended pattern has been removed is calculated. Then, a defective irregularity is detected using the obtained irregular form.
PTL 4 discloses a configuration in which pieces of one-dimensional measurement data of the tire surface are arranged in a circle and converted into two-dimensional measurement data, a convex hull filter is applied to the two-dimensional measurement data, measurement data that lies on a convex hull is extracted, and the extracted measurement data is converted back to one-dimensional measurement data.
It is extremely difficult to distinguish an irregularity that indicates a defect in a tire from a projection, such as a character or a pattern, which has been intentionally formed. With measurement data for only one line, a projection which has been intentionally formed cannot be accurately excluded from measurement data, and the projection may be determined to be an irregularity that indicates a defect.
According to any of PTL 1, PTL 2, and PTL 4, only one-dimensional data is obtained, and therefore, a projection that has been intentionally formed cannot be accurately detected, which is a problem. According to PTL 3, pieces of measurement data for two lines are obtained; however, the pieces of measurement data for the two lines lie spatially apart from each other. Therefore, a projection that has been intentionally formed cannot be accurately detected, which is also a problem.
Recently, a technique for acquiring measurement data for one line at a time by using a line laser has been developed. However, a system using a line laser is very expensive compared to a point-laser-type system, which is a problem.